1. Field of the Invention
The present invention relates to a method of quantifying a subject's wake or sleep state, and more particularly to a higher resolution method of determining when a subject is awake or asleep. The present invention additionally relates more specific applications of this method, and a system for making these quantitative measurements.
2. Technical Background
Nearly one in seven people in the United States suffer from some type of chronic sleep disorder, and only fifty percent (50%) of people are estimated to get the recommended seven (7) to eight (8) hours of sleep each night. It is further estimated that sleep deprivation and its associated medical and social costs (loss of productivity, industrial accidents, etc) exceed $150 billion dollars per year. Excessive sleepiness can deteriorate the quality of life and is a major cause of morbidity and mortality due to its role in industrial and transportation accidents. Sleepiness further has undesirable effects on motor vehicle driving, employment, higher earning and job promotion opportunities, education, recreation, and personal life.
Excessive daytime sleepiness (EDS) is a symptom describing an increased propensity to fall asleep, often during monotonous or sedentary activities. Though sometimes difficult, EDS vs. fatigue need to be differentiated. Fatigue or lethargy is where a subject senses a lack of energy or physical weakness and may not have an increased propensity to fall asleep at an inappropriate time. The underlying etiology of EDS generally falls into three categories: chronic sleep deprivation, circadian disorders (shift work), and sleep disorders. EDS is currently diagnosed via two general methods. The first is via subjective methods such as the Epworth and Standford Sleepiness Scale, which generally involves questionnaires where the patients answer a series of qualitative questions regarding their sleepiness during the day. With these methods, however, it is found that the patients usually underestimate their level of sleepiness or they deliberately falsify their responses because of their concern regarding punitive action, or as an effort to obtain restricted stimulant medication.
The second is via physiological based evaluations such as all night polysomnography to evaluate the patients sleep architecture (e.g., obtaining respiratory disturbance index to diagnose sleep apnea) followed by an all day test such as the Multiple Sleep Latency Test (MSLT) or its modified version, Maintenance of Wakefulness Test (MWT). The MSLT consists of four (4) to five (5) naps and is considered the most reliable objective measure of sleepiness to date. The MSLT involves monitoring the patient during twenty (20) to fourty (40) minute nap periods in two-hour intervals one and one half hour (1.5 hrs) to three hours (3 hrs) after awakenings to examine the sleep latency and the sleep stage that the patient achieves during these naps, i.e., the time it takes for the patient to fall asleep. A sleep disorder such as narcolepsy for example is diagnosed when the patient has a restful night sleep the night before but undergoes rapid eye movement sleep (REM sleep) within five (5) minutes of the MSLT naps. The MWT is a variation of the MSLT. The MWT provides an objective measure of the ability of an individual to stay awake.
While the MSLT and MWT are more objective and therefore don't have the same limitations as mentioned for the subjective tests, the MSLT and MWT have their own limitations. Both the MSLT and MWT require an all-day stay at a specialized sleep clinic and involve monitoring a number of nap opportunities at two hour intervals throughout the day. Further, the MSLT mean sleep latency is only meaningful if it is extremely short in duration (e.g., to diagnose narcolepsy), and only if the overnight polysomnogram does not show any sleep disordered breathing. Another problem with the MSLT mean sleep latency is the so-called “floor effect” where the sleep latency in the pathologically sleepy patients can be almost zero (0) minutes, i.e., the patient falls asleep almost immediately following turning off the light in the MSLT test. This type of result has a tendency to limit the diagnostic resolution of the test. Finally, studies have shown that the MSLT is not particularly suited for gauging the effects of therapeutic intervention. This was demonstrated in studies by Thorpy in 1992 and Van den Hoed et al. in 1981 showing no reliable reduction in sleepiness in patients given stimulant medications for narcolepsy.
The MWT was developed in 1982, in part, to address some of the short-comings of the MSLT method. The MWT eliminated the “floor effect” in the MSLT test shown in narcoleptic patients due to the instruction in the MWT test to the patient to stay awake. The MWT, however, created another problem at the other end of the sleep latency period called the “ceiling effect”. The “ceiling effect” is the tendency of less “sleepy” individuals to perform the MWT without falling asleep. In fact, the length of the MWT trial was lengthened from twenty (20) to fourty (40) minutes in 1984 because it was observed that patients with histories of excessive daytime sleepiness were too often able to maintain wakefulness for the twenty (20) minutes. In addition, while the MSLT and MWT are objective and “broadly” quantitative tests in that they both require the patient to fall asleep during the test and they measure the number of those incidents of sleep during the testing regiment, these tests are too costly and lack the degree of quantitative resolution necessary to easily permit measurement of effects of therapeutic intervention and degrees.
In recent years there have been a number of efforts to develop systems for detecting alertness and drowsiness by attempting to quantify the brain waves of a subject. Most of these systems have been aimed at the alertness monitoring field for alertness critical applications. Examples of these types of systems are as follows: Levin U.S. Pat. No. 6,167,298 discloses a device for monitoring and maintaining an alert state of consciousness for a subject wearing the device. With this device an alert mental state is maintained through monitoring of brain wave patterns to detect if a transition from an alert to a non-alert mental state is about to occur, or has occurred. If so, the device provides a stimulus until such time as an alert mental state, as assessed by the brain wave activity, is restored. Levendowski et al. U.S. Pat. No. 6,496,724 discloses a method of classifying individual EEG patterns along an alertness-drowsiness classification continuum. The results of the multi-level classification system are applied in real-time to provide feedback to the user via an audio or visual alarm, or are recorded for subsequent off-line analysis. Kaplan et al. U.S. Pat. No. 5,813,993 discloses an alertness and drowsiness detection and tracking system. The system claims improved performance by preserving and analyzing brain wave signal components at frequencies above 30 Hz.
Most of the methods, systems or devices currently on the market either provide a qualitative means for analyzing for excessive daytime sleepness or more specifically for sleep disorders, or a semi-quantitative means for classifying a subjects state of alertness. None of the above mentioned methods, systems or devices provide a quantitative means of measuring and determining whether an individual suffers from excessive daytime sleepiness and more specifically from a sleeping disorder, particularly one in which the analysis and measurement are capable of being provided in a short time duration and at low cost to the patient or insurance company. Furthermore none of the above mentioned methods provide a method with the sensitivity for very accurately quantifying the degree of sleepiness or alertness, i.e., micro-arousals or micro-sleep. It is therefore an object of the present invention to provide a quantitative method of analysis wherein it can be determined whether a patient exhibits excessive daytime sleepiness. It is still another object of the present invention to provide a method of more accurately determining the wake-sleep transition. It is still another object of the present invention that this method be inexpensive and/or of short time duration. It is still another object of the present invention that a patient's therapeutic treatment can be more accurately determined based on these methods, and can subsequently be adjusted accordingly based on a subsequent test of the patient.